Added the BackgroundParent interface

Parents are now able to draw backgrounds for their children. This
means we can now have elements inside other elements that aren't
restricted to one background color.

elements/label.go

@@ -200,13 +200,10 @@ func (element *Label) updateMinimumSize () {
 }
 
 func (element *Label) draw () {
+	element.core.DrawBackground (
+		element.theme.Pattern(tomo.PatternBackground, tomo.State { }))
+
 	bounds := element.Bounds()
-
-	pattern := element.theme.Pattern (
-		tomo.PatternBackground,
-		tomo.State { })
-	pattern.Draw(element.core, bounds)
-
 	textBounds := element.drawer.LayoutBounds()
 
 	foreground := element.theme.Color (